This invention relates to a thermal and optical camouflage comprising a sheet material which is provided with a camouflage-coloured surface and which provides means for controlling heat emission to more certainly screen heat radiating machines and other objects by creating thermal emission patterns similar to that of the nature of the locale.
Leaf-cut camouflage material provided with slits as described, for example, in U.S. Pat. No. 3,069,796, have been in use for a long time as ultraviolet, visual and near IR camouflage. It is usually fixed to a supporting net. Swedish camouflage nets are generally garnished with equal pieces of material which are stretched into hexagons being about 40 cm across and laid side by side, and glued on. In other designs there are used more irregular variously coloured figures which are fastened to the net by means of clamps and the like. The garnishing material is normally of matte-surfaced polyvinyl chloride film.
These camouflage nets function very well as far as ultaviolet, visual and near IR camouflage is concerned because the outlines of covered objects are disguised and a picture is obtained which differs very little from the natural environment provided that the colour scheme tolerably harmonizes with what exists in the nature where they are to be used. Thus, there are manufactured, for example, special nets for woodland natural environment, for desert natural environment and for snow natural environment, which of course have considerably different colour schemes.
However, these camouflage nets do not camouflage particularly well against reconnaissance with instruments having detectors for infrared radiation in the thermal ranges 3 to 5 .mu.m and 8 to 14 .mu.m. For such radiation the known camouflage nets mentioned have far too poor camouflage properties since they appear as surfaces having even temperature and emit infrared radiation entirely determined by their temperature. This in turn is greatly determined by the temperature of underlying objects and meteorological conditions such as solar warming.
It is an object of the invention to provide a camouflage material having improved camouflage effect in the above-mentioned thermal infrared wavelength ranges, and which in the UV-visible and photographic IR-wavelength ranges has as good a camouflage property as the above-mentioned known ones.
This purpose is attained according to the invention by the fact that in the camouflage material there is included coloured plastics on both sides of a reflecting metal layer, the plastics including, at least partially, polyethylene or polypropylene or other polymer which is highly transparent to thermal infrared waves and therefore amenable to controllable emissivity.
The reflecting layer in the camouflage material has a result that the side of the camouflage which is visible from the outside with respect to radiation will be insulated from the camouflaged objects so that the equilibrium of radiation will be of greater similarity to the natural environment and the always cold sky. Because certain portions of the surfaces contain polyethylene or other thermal infrared transparent polymer there is also obtainable temperature contrasts over the exposed surface of the camouflage material because polyethylene together with the underlying low emissivity metal surface can present a particularly low thermal emission factor. In an infrared picture these portions will appear colder than they are, and therefore a thermal-picture will appear which shows a varied thermal pattern even though the temperature is uniform. It is thus desirable that portions of the surface are polyethylene and portions of other plastic material such as polyvinyl chloride which exhibits different emissivity. Polypropylene and certain other plastic materials, e.g., butyl rubber, have emissivity properties similar to those of polyethylene.
The plastic surface materials may suitably be coloured throughout with low-absorbing camouflage pigments. The surfaces should be structured matte in order to reduce specular reflectivity.
As the camouflage material is leaf-cut to only a limited extent, the camouflage material will substantially cover and be useful for thermal camouflage even if the camouflaged objects have temperatures substantially higher than that of the environment. Hot objects should however be specially insulated by blankets or mats so as not to be visible, and hot engines, generators etc. should be provided with particular temperature screenings and preferably be air-cooled, and the warmed cooling air should be carried off in a suitable manner in order to prevent the camouflage or other exposed material from being warmed. Hot air is not visible in IR pictures unless it has been allowed to warm opaque objects, since air has low emissivity.
The favourable effect of polyethylene or polypropylene or copolymers having a considerable content of either of these, an effect which is at first surprising, is associated with the same phenomenon, that is to say that material of low absorptivity has also low emissivity. This property is most pronounced in polyethylene.
According to the invention it is therefore to the purpose to mix areas of polyethylene or similar low-absorbing plastics with areas of other plastic materials, such as polyvinyl chloride, which have different emissivity characteristics in the surface of camouflage of the above-mentioned type. For this to be most useful, care should be taken to ensure that the heat radiation coming from the inside is minimized and that the radiation coming from the outside becomes more significant to the sensor, this desirable effect being accomplished by building in a reflecting metal layer. If desired, this layer may be in the form of a mosaic of electrically unconnected areas as well as thin in order to control radar reflection, also. The thermal pattern may be made up by means of continguous patches having differing emissivities.
The manufacture of the camouflage material used for the invention may be performed in different ways. One possibility is to coat a surface of blown film polyethylene with a vapour-deposited layer of metal, upon which is applied a plastisol of polyvinyl chloride or a film of such material. Care should be taken that the camouflage material has a matte surface, in order to enhance the camouflage effect.
When leaf-cut camouflage materials of the type known per se are drawn out, portions of the camouflage material will rise from its plane and form a three-dimensional structure. This is virtually done almost irreversibly because considerable care is necessary to fold up the material again into a flat material--it is necessary to turn all the parts having turned the back side up when being drawn out. In accordance with a particular embodiment of this thermal camouflage, this effect can be utilized by initially stretching the material, which results in a three-dimensional leafage which can then be pushed laterally together. A three-dimensional leafage with smaller holes, which can in fact become exceedingly small, will then be obtained. It is possible to push the material together to such an extent that the leafage formed covers a surface which is smaller than the surface covered initially by the unstretched material. A very fluffy three-dimensional structure is then obtained which has a high covering and thermal screening effect which enhances the thermal camouflage properties of the camouflage material as above described. This fluffy structure may be affixed to a supporting web by means of an adhesive or by mechanical means such as clamps or sewing.
In accordance with a particular embodiment, camouflage materials are made which are, so to say, emissivity patterned, i.e., have crazy quilt designs of differing areas of emissivity, on one or both sides. Such a pattern may be effected by selecting materials having varying emissivity; it has already been mentioned that through polyethylene an emission magnitude can be obtained which is lower than that of a black body of the same temperature. The thickness of the polyethylene layer can also be varied so that different portions have different thicknesses and thus different emissivities. To the same end, different areas of the surface may be different mattness. By pigmentation the infrared pattern can also be varied. An active pigment to increase the emissivity in the wavelength ranges of particular interest (3 to 5 and 8 to 14 .mu.m) is carbon black.